According to the international Committee on Data for Science and Technology (CODATA), the charge radius of the proton is 0.8768(69) fm. Few researchers would give that number much thought if not for measurements in 2010 and 2013 that yielded a radius 4% smaller than and 7.2 standard deviations distant from the CODATA value. Randolf Pohl of the Max Planck Institute of Quantum Optics in Garching, Germany, and colleagues obtained the curiously low radius after analyzing the energy-level shifts of muons orbiting hydrogen nuclei. With a mass 207 times that of the electron, a muon has a tighter orbital that more closely overlaps the nuclear charge distribution, which makes the negatively charged particle a useful tool for probing nuclear dimensions. The discrepancy between the results of muon-based and other experimental investigations has come to be known as the proton radius puzzle.
Now Pohl and his colleagues have used the same technique...
